In the production of aluminates, there are formed aluminate liquors which, in spite of the relatively low gallium concentration (0.2 to 0.3 grams per liter), have proved to be suitable sources for the industrial production of gallium.
The recovery of gallium can take place by a number of conventional processes. Most of the electrolytic techniques involve the deposition of gallium on mercury cathodes. The mercury or sodium amalgam cathodes which are used for this purpose have the advantage, over solid metal cathodes of a single metal, that they have a high hydrogen overvoltage so that gallium deposits with high efficiency.
Of course, in spite of the aforementioned advantages, the use of mercury cathodes had had limited applicability because of the health and environmental hazards which are produced when the mercury is released into the environment as is common with the conventional techniques.
In alkali media, thus in aluminate liquors, the electrode deposition potential of gallium is more negative than -1.6 volts. With solid metal cathodes this electrode potential cannot be maintained or can be maintained only fleetingly. The potential of the cathode during electrolysis changes as a result of the deposition of electropositive impurities from the aluminate liquor and as a result of the shielding effect of reducing organic substances these effects tending to make the cathode more positive. Within increasingly positive potentials, the deposition of gallium does not take place to completeness since the cathode operates as a hydrogen generating electrode. In practice it is found that gallium does not deposit on solid metal electrodes because of this interplay of effects.